1. Field of the Invention
This invention relates to the use of metal oxide-polymer composite particles as external additives for electrophotographic toner.
2. Description of the Related Art
Electrophotographic image formation comprises uniform charging of the surface of a photoreceptor drum or belt; exposure of the photoreceptor surface to light and formation on the photoreceptor surface of a charge pattern, i.e., a latent image, that mirrors the information to be transferred into a real image; developing the latent image with electrostatically charged toner particles comprising a colorant dispersed in a binder resin; transferring the developed toner onto a substrate, e.g. paper; fusing the image onto a substrate; and preparing the photoreceptor surface for the next cycle by erasing the residual electrostatic charges and cleaning the remaining toner particles.
Toners for use in electrophotography and electrostatic printing include a binder resin and a colorant, and may further include a charge control agent, an offset-preventing agent, and other additives. External toner additives such as metal oxide particles are often combined with toner particles in order to improve selected properties of the toner particles, including fluidity, transferability, fixability, and cleaning properties. A variety of external additives may be used in a single toner composition to enhance different properties of the toner. For example, some additives may be selected to improve chargeability, i.e., tribocharge. Others may be selected to improve cleaning performance or humidity resistance. Of course, it is preferable that a toner additive optimized for one function is not detrimental to the functions imparted by the various additives.
One function imparted by the toner additives is spacing and maintenance of fluidity. If the toner particles adhere to one another, they will not flow as well; additives serve to reduce the cohesion of the toner powder. The additive particles tend to be hard. The toner, on the other hand, is formed from softer polymers and is a coherent powder. The resulting agglomeration of toner particles is detrimental to both the operation of the electrophotography apparatus and to the print quality. Indeed, as manufacturers have sought to reduce the energy required to produce a printed page, they have turned to softer polymers (i.e., lower Tg polymers) to reduce the amount of heat required to fuse the toner to a substrate. However, the hard additive particles can become embedded in the soft toner particles, reducing the effectiveness of the additive. Increasing the size of the additive particle reduces embedding; however, the larger particles are also heavier and exhibit a higher rate of drop-off from the toner particle. Of course, additive particles that drop off the toner cannot serve their function as part of the toner composition. Thus, it is desirable to have a toner additive that serves as a spacer between toner particles that both exhibits limited embedding in the toner particles and limited drop-off without adverse impact on the tribocharge characteristics of the toner.